308 Green Chemistry, 2nd ed
- Carbohydrate, which has the general formula of approximately CH 2 O.
Carbohydrate is the biomass that is produced initially as glucose sugar
from water and carbon dioxide during photosynthesis. It is contained in the
structural parts of plants as cellulose, a biopolymer. - Lignin, a biological polymer with a complex structure, which occurs with
carbohydrate cellulose in woody parts of plants, binding fibers of cellulose
together. Relatively few uses have been found for lignin, and it poses impurity
problems in extracting cellulose for feedstock use. - Lipid oils extracted from seeds, including soybeans, sunflowers, and corn.
- Hydrocarbon terpenes produced by rubber trees, pine trees, and some other
kinds of plants. - Proteins, produced in relatively small quantities, but potentially valuable as
nutrients and other uses.
Biological materials used as sources of feedstocks are usually complex mixtures,
which makes separation of desired materials difficult. However, in some biological starting
materials nature has done much of the synthesis. Most biomass materials are partially
oxidized as is the case with carbohydrates, which contain approximately one oxygen
atom per carbon atom (compared to petroleum hydrocarbons which have no oxygen).
This can avoid expensive, sometimes difficult oxidation steps, which may involve
potentially hazardous reagents and conditions. The complexity of biomass sources can
make the separation and isolation of desired constituents relatively difficult.
There are several main pathways by which feedstocks can be obtained from biomass.
The most straightforward of these is a simple physical separation of biological materials,
such as squeezing oil from oil-bearing biomass or tapping latex from rubber trees. Only
slightly more drastic treatment consists of extraction of oils by organic solvents. Physical
and chemical processes can be employed to remove useful biomass from the structural
materials of plants, which consist of lignocellulose composed of cellulose bound together
by lignin “glue.”
Carbohydrates are perhaps the most likely candidates as feedstocks for chemical
processes. Carbohydrates come in several forms. Sucrose sugar, C 12 H 22 O 11 , can be
squeezed from sugar cane as sap and can be extracted from sugar beets and sugar cane
with water. The exceptional photochemical productivity of sugar cane makes sucrose
from this source an attractive option. Larger amounts of carbohydrates are available in
starch, a polymer of glucose readily isolated from grains, such as corn, or from potatoes.
An even greater source is found in cellulose, which occurs in woody parts of plants. It
is relatively easy to break down starch molecules with the addition of water (hydrolysis)
to give simple sugar glucose. Breaking down cellulose is more difficult, but can be
accomplished by the action of cellulase enzymes.
Lipid oils are extracted from the seeds of some plants. Volatile solvents, most
commonly the 6-carbon straight-chain alkane n-hexane, C 6 H 14 , are used to extract oils.